HIV-1 fitness is a complex parameter defined by the adaptability of the virus to a given environment. Over the past four years we have studied the ex vivo fitness of HIV-1 in various primary human cells and have explored possible relationships with disease progression as well as global and temporal evolution of the virus in the human epidemic. In this proposal we will focus on changes in replicative HIV-1 fitness during the natural history of disease and following discrete transmission events in subtype A, C, and D infected patients. Over the past three years, we have analyzed samples from over 266 Zimbabwean and Ugandan women recruited within three months of acute infections and have followed their progression every three months thereafter. This cohort also provides an excellent opportunity to compare the affects of HIV-1 subtype on infection and subsequent disease outcomes. As a lead up to this study, we have published fairly exhaustive comparisons on the fitness of HIV types (1 or 2), groups (M and O), subtype (A through F), and recombinant forms and found the order of ex vivo HIV replicative fitness to be: HIV-1 group M (subtypes A, B, D, F, CRF01_AE, CRF02_AG, CRF012_BF) >subtype C >HIV-2 " HIV-1 group O. Based on preliminary data that shows slower disease progression in subtype C infections and that subtype C virus is intrinsically less fit, our study will address if these two observations are linked and if subtype C represents an attenuated HIV-1 form. These hypotheses will be tested in the following specific aims. Specific aim 1: To establish a rapid yeast-based env cloning system to produce chimeric env viruses. To further examine if ex vivo HIV-1 fitness maps to the env gene and is controlled by the HIV-1 entry process into host cells. Specific aim 2: To explore the relationships between HIV-1 fitness/diversity at acute/early infection Specific aim 3: To examine changes in viral fitness, genetic diversity, and other clinical correlates during disease progression. To relate changes to subtype.